U.S. Pat. Nos. 6,586,141 and 6,673,497 and summarizing publication L. B. Glebov, V. I. Smimov, C. M. Stickley, I. V. Ciapurin, New approach to robust optics for HEL systems, Laser Weapons Technology III, Proceedings of SPIE, 4724 (2002) pp. 101-109 teach how to make diffractive optical elements from photosensitivity photo-thermo-refractive (PTR) glass with efficiency exceeding 95%.
These diffractive optical elements are used as spatial filters, attenuators, beam splitters, beam sampler, beam deflector controlled by angular positioning of grating or spectral scanning of the incident beam, selector of particular wavelengths, also known as notch filter or add/drop element, spectral shape former, also known as gain equalizer, spectral sensor, also known as wavelocker or wavelength meter, angular sensor, also known as angular pointer, Bragg spectrometer, also known as spectral analyzer, and selectors of transverse and longitudinal modes in laser resonators. All these diffractive optical elements are based on the use of specific angular and spectral selectivity of Bragg gratings.
A basic theory of such gratings was developed by H. Kogelnik, as described in “Coupled wave theory for thick hologram gratings”, Bell System Tech. J. 48, (1969), pp. 2909-2945 and was used in the U.S. Pat. Nos. 6,586,141 and 6,673,497 (previously cited) for modeling of spectral and angular selectivity of both reflecting and transmitting gratings. It was shown that spectral and angular selectivity of Bragg gratings could be controlled by proper selection of their basic parameters which include spatial frequency, refractive index modulation, and thickness. The range of variations of Bragg gratings parameters (spectral or angular selectivity) is very wide and covers most of the requirements of different optical and laser systems. However, the shape of the element is predetermined by the periodical modulation of a refractive index. Thus, the relatively narrow top of a selectivity function and the presence and positions of side lobes are usually considered as intrinsic drawbacks of volume Bragg gratings that could not be avoided.
Chirped gratings with variable period, or spatial frequency, are well known in optical science and are widely used for spectral filtering and analysis. However, the main part of the chirped gratings is made in fiber geometry. Chirped gratings were used for narrow band spectral filtering as disclosed in Songyang Li, Nam Quoc Ngo, Swee Chuan Tjin and Le Nguyen Binh, “Tunable and switchable optical bandpass filters using a single linearly chirped fiber Bragg grating,” In press Optics Communications, (2004). The chirped gratings were also used for laser wavelength stabilization and dispersion compensation as described in Xiaoke Yi, Chao Lu, Xiufeng Yang, Wen-De Zhong, Fang Wei, and Yixin Wang, High-birefringence linearly chirped grating based optical device for PMD, Opt. Expr. 11, (2003) p. 2634; in Pei Li, Jian Shuisheng, Yan Fengping, Ning Tigang and Wang Zhi, Long-haul WDM system through conventional single mode optical fiber with dispersion compensation by chirped fiber Bragg grating, Optics Communications 222, (2003) p. 169; for gain flattening as described in Audrey Elisa Lobo, James A. Besley, and C. Martin de Sterke, Gain-Flattening Filter Design Using Rotationally Symmetric Crossed Gratings, Journal of Lightwave Technology 21, (2003) p. 2084; for equalizing gain as described in Martin Guy, and Francois Trépanier, Chirped Fiber Bragg Gratings Equalize Gain, Laser Focus World, Supplement issue, (2001), p. 77 and multi-wavelength signal demultiplexing as described in E. Simova, M. Kavehrad and K. Stoev, Wavelength demultiplexing by chirped waveguide gratings, Optics Communications 134, (1997) p. 330.
An example of the use of volume chirped grating recorded in Fe:LiNbO3 for side lobes suppression in spectral filters described in Seunghoon Han, Bong-Ahn Yu, Seunghwan Chung, Hwi Kim, Jungwook Paek, and Byoungho Lee, Filter characteristics of a chirped volume holographic grating, Optics Letters 29, (2004) p. 107. The use of volume chirped gratings is restricted by the lack of available photosensitive materials which provide high sensitivity, low losses, and stability of volume in the processes of exposure and development. The last feature is extremely important for chirped gratings because of necessity for precise control of spatial distribution of grating period.
It is important to note that modeling of gratings with variable period is difficult with the use of conventional Kogelnik's theory of coupled waves. This is why matrix approach disclosed in S. Huang, M. LeBlanc, M. M. Ohn, and R. M. Measures, Bragg interrogating structural sensing, Appl. Opt. 34, (1995) p. 5003 and in Gabriel Cormier, Roger Boudreau, and Sylvain Thériault. Real-coded genetic algorithm for Bragg grating parameter synthesis, J. Opt. Soc Am. B 18, (2001) p. 1771 was used for modeling of chirped gratings.
Recent advance in laser aided material processing causes increased demands on high peak power femtosecond lasers. The use of chirped gratings for stretching and compression of femtosecond laser pulses allows increasing of pulse energy. The most contact design of femtosecond laser is based on all fiber geometry, where chirped fiber gratings are used to compress and decompress pulses A. Galvanauskas, M. E. Fermann, D. Harter, K. Sudgen, and I. Bennion, All fiber femtosecond pulse amplification circuit using chirped Bragg gratings. Appl. Phys. Lett. 66 (1995) 1053. One of the advantages of chirped fiber gratings for this use is that you can obtain good beam quality especially when identical gratings are used for the compression and the decompression. This allows the output beam to be more identical to the input beam, considering the shape for instance.
One of the drawbacks of chirped fiber gratings is that those fibers cannot tolerate high power density which is necessary for high power amplification. This limitation is due to the small aperture resulted in high power density and, therefore, in low damage threshold of fibers. The best pulse energy that so far has been obtained for all-fiber femtosecond system is <100 nJ A. Galvanauskas, D. Harter, S. Radic, and G. P. Agrawal, High-energy femtosecond pulse compression in chirped fiber gratings, in Conference on Lasers and Electro-Optics, Vol. 9, 1996 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1996), pp 499-500.
Higher energies can only be achieved by means of a system of gratings or prisms that are added to recompress the pulses, which seriously enhance complexity of laser systems as described in C. M. Gonzalez Inchauspe, O. E. Mart nez, Aberration compensation of a curved diffraction grating stretcher for femtosecond chirped-pulse amplification. JOSA B 14 (1997) p. 2696 and in G. Lenz, K. Tamura, H. A. Haus and E. P. Ippen, All-solid-state femtosecond source at 1.55 um. Opt Lett. 20 (1995). Another disadvantage of such high requirements for alignment of surface grating and other optical elements in compression blocks like telescopes lenses, complicated mirrors that are needed as disclosed in J. Limbert, T. Schreiber, T. Clausnitzer, K. Zollner, H-J. Fuchs, E. -B. Kley, H. Zellmer, A. Tunnermann, High-power femtosecond Yb-doped fiber amplifier. Optc Expr 10 (2002). Small alignment errors in the positioning of all these elements can cause frequency dispersion as described in Terrance J. Kessler, Joachim Bunkenburg, Hu Huang, Alexei Kozlov, David D. Meyerhofer, Demonstration of coherent addition of multiple gratings for high-energy chirped-pulse-amplified lasers. Optics Letters 29 (2004) pp. 635-637. Moreover, those systems cannot be compact because of large distance needed between the two gratings or prisms used for compression. A limiting factor of surface diffraction gratings is their low optical damage threshold (2 J/cm2 in 1 ns pulse) which results in very large apertures in the range of tens of centimeters. However, manufacturing of large aperture gratings is challenging, so instead of one large coherent addition of several gratings can be used as disclosed in G. P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, Calif., 1995. It makes such technology extremely complicated and expensive.
Another serious limitation of surface-diffraction-grating compressors is associated with the restricted average-power handling capacity. Existing diffraction-grating compressors have not been able to tolerate more than 100-W of average power, with tens of watts being a typical limit. With fiber laser power exceeding 1-k level, this limitation is becoming the main hindrance on the path of power scaling of ultrashort-pulse laser technology.
Previously, solution of reducing complexity of CPA arrangement through the use of chirped volume Bragg gratings have been proposed, in order to overcome limited mode-area of chirped fiber Bragg grating compressors as disclosed in U.S. Pat. No. 5,499,134 issue on Mar. 12, 1996 to Galvanauskas et al,. However, no suitable method of achieving chirped volume gratings have been identified there. In fact, experimental attempt to implement such gratings through UV-written photosensitive-glass gratings identified main difficulty of achieving required performance as described in A. Galvanauskas, A. Heaney, T. Erdogan, D. Harter, Use of volume chirped Bragg gratings for compact high-energy chirped pulse amplification circuits, in Conference on Lasers and Electro-Optics, vol. 6, 1998 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1998), p. 362. It was demonstrated that due to the exponential decay of the writing-UV-beam intensity (due to writing-beam absorption during writing process) the resulting gratings are highly inhomogeneous in the depth direction, producing highly spatially distorted pulse-compressed beams. This, on one hand, severely limits the attainable volume-grating aperture size to no more than 100-300 um, and on the other hand, produces unacceptable beam quality loss.
Thus, the main approach of the proposed invention is a combination of properties of high efficiency volume Bragg gratings in PTR glass, which allow achieving very large sizes (tens of millimeters) of both transverse apertures and in depth direction with highly homogenous spatial grating profile, and ideology of stretching and compression of short pulses for power amplification by chirped fiber gratings.
Unique properties of gratings recorded inside PTR glass enable creation of very large apertures with homogeneous transverse spatial profile, thus eliminating any significant beam distortions and allowing to scale pulse energies into multi-mJ energy range and higher. Furthermore, intrinsic ability of PTR glass to withstand high average laser powers (our recent tests indicated no damage for 0.5-kW laser power focused into 350-um diameter spot in PTR glass) provides with unique method of implementing high power (from 100 W to multi-kilowatt level) femtosecond technology, which has not been attainable with any other previously demonstrated compressor technology.